CHICAGO — Like many Web users of a certain age, Ralph Dahm doesn't have a profile page at www.MySpace.com or participate in the gossipy give-and-take at www.Facebook.com.

"That's more for the kids," said the 55-year-old job recruiter for technology companies. But that doesn't make Dahm a reluctant participant in the social networking trend exploding across the Internet.

Indeed, he has more than 5,200 contacts through www.Linkedin.com, a social tool for business acquaintances that he actively maintains each day. And, unlike the kids who collect thousands of friends on MySpace in an effort to appear popular, Dahm can tell you a little bit about each of his contacts.

"No. 4,972 is a guy at Volkswagen Credit," Dahm said. "I first contacted that person on July 25. I may not have spoken with him but I keep up to date with what he's doing."

The trend tabbed "social networking" is vastly broader than MySpace, and its components are quickly being incorporated into hundreds if not thousands of Web sites, ranging from autos to music and even shopping.

The sites are defined by content —photos, product reviews and the occasional rant — that users contribute, helping build a sense of community and participation.

Social networking represents an evolution of the Web, from a static place where people searched for news and information into more of a gigantic coffeehouse, where people gather to share their opinions and swap photos, stories and videos. It's a two-way Web.

The evolution exploded onto public consciousness after News Corp. paid $650 million for MySpace last year. Now Yahoo is reportedly considering a purchase of Facebook.com, a smaller MySpace rival, for about $1 billion.

That has many people wondering if such a high price for a site with fewer than 15 million unique visitors in August is worthwhile, considering the very real possibility that another trendy site is lurking in the background.

"MySpace is worth a lot because it's the leader in the field. They have a lot of community and a lot of users," said J.B. Pritzker, a partner with New World Ventures, an Evanston, Ill., venture capital firm.

"But if the question is, is the technology replicable, the answer is yes.

"On a financial basis, in my book, you can't possibly justify $1 billion for Facebook," he said.

The tools that have made MySpace and Facebook so popular are rampant across the Web.

At www.Boompa.com, for instance, auto enthusiasts share photos of cars, motorcycles and their motor-loving credentials in what site operators call an online garage. At www.Yelp.com, users post their thoughts on Chicago-area health clubs, restaurants, spas and nightclubs, among other things. And at www.Tripadvisor.com, users make travel plans based on reviews from customers who stayed at a city's top hotels and ate in their restaurants.

"Trip Advisor is starting to gain some acceptance," said Bill Tancer, general manager of global research for Hitwise, a Web tracking firm. "It now matters more to major hotel chains how many bubbles or whatever it gets on Trip Advisor than stars from AAA ratings."

George Luhowy, a publisher of computer-related information in Boston, tracks about 425 Web sites that use some sort of social networking aspect. "That's probably doubled in the last three months," he said, "and there are probably less than 50 or so in our pipeline" that the company will soon add to a database it sells to institutions that monitor Web trends.

Yet despite the growth of sites that rely on user-generated content, none has the audience of MySpace or Facebook.

In August, MySpace had 55.7 million unique visitors, nearly four times the 14.7 million Facebook visitors, according to figures from ComScore Media Matrix.

Microsoft Corp.'s Windows Live Spaces follows with 9.7 million users, www.Xanga.com has 8 million and www.Flikr.com, a photo site Yahoo bought last year for an undisclosed amount, has 5.5 million users.